On FreeBSD, add -pthread to ASan dynamic compile flags for tests

Otherwise, lots of these tests fail with a CHECK error similar to:

==12345==AddressSanitizer CHECK failed: compiler-rt/lib/asan/asan_posix.cpp:120 "((0)) == ((pthread_key_create(&tsd_key, destructor)))" (0x0, 0x4e)

This is because the default pthread stubs in FreeBSD's libc always
return failures (such as ENOSYS for pthread_key_create) in case the
pthread library is not linked in.

Reviewed By: arichardson

Differential Revision: https://reviews.llvm.org/D85082